Passive millimeter wave (MMW) imaging has many applications such as remote sensing of the Earth's resources, aircraft landing in optically obscure weather, and security point inspection of concealed weapons in humans. The underlying principle is the measurement of Planck's blackbody radiation of materials at millimeter wavelengths. The main advantage of passive MMW imaging is that it provides information about ground-based targets under all weather conditions; optical systems (visible and IR), on the other hand, require clear atmospheric conditions for reliable operation. For example, the atmospheric attenuation at MMW frequencies is 0.07 to 3 dB/km in drizzle and fog, whereas it is one to three orders of magnitude higher at optical frequencies (exceeding 100 dB/km in foggy conditions). (See, e.g., L. Yujiri, M. Shoucri, and P. Moffa, “Passive millimeter-wave imaging,” IEEE Microwave Magazine, September (2003); R. Appleby and R. N. Anderton, “Millimeter-wave and submillimeter-wave imaging for security and surveillance,” Proc. IEEE, 95, 1683-1690 (2007); A. H. Lettington, D. Dunn, M. Attia, and I. M. Blankson, “Passive millimeter-wave imaging architectures,” J. Optics A: Pure and Applied Optics, 5, S103-S110 (2003)). Excellent image contrast is obtained in outdoor environments due to cold sky-reflected radiation by targets. For example, the apparent temperature of the sky at 94 GHz is 70K in comparison to 220K at infrared wavelengths. Even at the same ambient temperature, there exists variation in MMW thermal contrast of objects because of emissivity differences of objects at these wavelengths, e.g., the emissivity of metal is ≈0, water 0.4, wood 0.4, and concrete 0.8. (M. R. Fetterman, J. Grata, G. Jubic, W. L. Kiser, Jr., and A. Visnansky, “Simulation, acquisition, and analysis of passive millimeter-wave images in remote sensing applications,” Optics Express, 16, 20503-20515 (2008).) As a result, signal “washouts” do not occur because the apparent temperature between the background and the object are rarely similar.
In addition to imaging, passive millimeter waves can be used to obtain spectroscopic signatures of chemicals based on molecular rotational energy transitions. With a 16-channel filter bank in the 146-154 GHz band, the 150 GHz spectral line of nitric oxide from a test stack at a distance of 600 m from the radiometer has been measured. (N. Gopalsami, S. Bakhtiari, T. W. Elmer, and A. C. Raptis, “Application of Millimeter-Wave Radiometry for Remote Chemical Detection,” IEEE Trans. on Microwave Theory and Techniques, 56, 700-709 (2008)). While imaging can provide broad area search of facilities for certain observables such as structural changes, traffic, and effluent heat, the spectroscopy system can provide more specific signatures of effluent chemicals from exhaust stacks. Millimeter wave radiation allows for rotational spectroscopy of polar molecules, so it can provide fingerprint signatures of chemicals emanating from material processing facilities. FIG. 1 is an example of an outdoor image that was obtained with a prior system.
A major disadvantage of such a single-pixel detector system is the long scanning time for image acquisition. For example, a 100×100 pixel image with 1 s integration time per pixel requires a minimum of 2 h 47 min. With such a long imaging time, the imager's value is diminished for applications involving imaging of nonstationary objects or for real-time or near real-time applications. Stemming from the need for faster imaging, there has evolved the concept of compressive sensing which has the potential in reducing the image acquisition time by a factor of 10 or more.